As wafer size increases and as device geometry becomes smaller, particulate detection and control becomes ever more important in semiconductor processing. Monitoring of particulate levels is important in processes which take place in an environment of air at atmospheric pressure, for example exposure of photoresist patterns, and for processes which take place in a vacuum chamber, for example deposition of metal films. Particulate contamination can be reduced for processes which take place in an environment of air at atmospheric pressure by the use of so called clean rooms which employ air filtration systems. Even with air filtration systems, however, processing equipment employs moving parts which generate particles and monitoring of particulate levels is therefore desirable for early detection of system breakdowns which produce excessive particulate levels.
One prior art method for detecting airborne particles is shown in FIG. 1. Sampled air (indicated by arrow 5 in FIG. 1) is drawn through narrow transparent tube 6 by a vacuum pump (not shown) attached to end 6a of cylindrical tube 6. Monochromatic light 1 from a laser (not shown) or white light from a lamp (not shown) is focused by lens 2 to form a focused beam 3 which passes through transparent tube 6 at a selected point along the tube. Light scattered from particles in sampled air 5 drawn through tube 6 which passes through beam 3 is detected by detector 7. Alternatively an opening (not shown) in tube 6 and an air sheath may be provided so that the focused beam passes through the opening in the tube. Detector 7 contains a photomultiplier and its construction is well known in the art. The scattering intensity is roughly proportional to particle size. Such systems commonly detect particles having a mean diameter in a range between 0.1 microns and 7.5 microns and in principle even smaller and larger particles can be detected using the above system.
This prior art particle monitoring device has several drawbacks:
(1) It essentially samples air from a point, i.e. the point of the opening of the tube, and does not provide an adequate measure of particulate contamination over a wider spacial region. Often in a semiconductor processing environment, moving parts of various machinery may produce particles that will not be detected by sampling at a particular point. Thus prior art particle monitoring devices do not adequately monitor particles from multiple or distributed sources.
(2) The prior art monitoring system works in air but not in a vacuum chamber since it requires a flow of air to carry the particles.
(3) Particles may stick to the sides of tube 6 and then become airborne again at a later point in time thereby creating a delay effect.
(4) The physical end 6b of the tube must be placed physically close to the point being monitored which may interfere with other portions of the processing system.